The major events leading to the activation of a skeletal muscle have been worked out, but there are further details that are important to know if one is to understand this system fully. The aims of the present proposal are to study some of the excitation-contraction (e-c) coupling steps not well-defined yet, and some mechanisms by which they are controlled. These are: I. The mechanisms that produce mechanical fatigue. We would like to determine in fatigue fibers: the depth and rate of propagation of the T-tubule action potential, the differences in the ability of action potentials and steady-state depolarizations to release Ca ions, the duration of the active state and the twitch-tetanus tension ratio, and changes in fiber capacitance. II. The influence of extracellular and intracellular pH (pHe and pHi) on the contractility. Our aim is to determine the effect of low pHe with non-penetrating buffers on: the twitch and tetanic forces, the Ca ions release from the SR and the actomyozin interaction, the contractility at 3 degrees C, pHi in the above conditions, and the action potential characteristics at low pHe. III. Intracellular ionic movement analysis during mechanical activation with electron probe techniques. Our aim is first to obtain better resolution Ca X-ray maps in muscles in which K has been replaced with Rb, then to measure: the changes in the composition of terminal cisternae during the early phase of activation when the SR calcium pump has been reduced, the subcellular distribution of Ca ions in muscle following the contraction and the maximal amount of Ca released at 3 degrees C through either tetanus or K-contractures.